Not Applicable.
Not Applicable.
1. Field of the Invention
The present invention relates generally to welding power supplies. More specifically, the present invention relates to an improvement in the conduction of freewheeling current.
2. Background of the Invention
Welding power supplies are typically stand-alone units which receive a standard line voltage and provide a usable welding power at a welding output. The welding power may be alternating current (AC) or direct current (DC), constant current or constant voltage, three-phase or single-phase, and may include a wide range of amperages, all depending upon operator-selected inputs. Various power and control circuitry is used to shape and time the welding power based upon the operator-selected inputs.
Many welding power supplies utilize switches or gating devices, such as silicon-controlled rectifiers (SCRs), to control the amount of power provided at the welding output. An SCR is a three-terminal device which provides current from an anode to a cathode in response to a current provided to a gate when the device is forward-biased. SCRs are in wide usage in welding power supplies. A control circuit is used to drive the gate to control the SCR.
A standard topography for a power conversion circuit for a welding power supply is shown in FIG. 1. An AC source 10 provides an AC current through a transformer 12 to a bridge rectifier 14. Bridge rectifier 14, comprising four SCRs, rectifies the AC current and provides it to a welding output 16. The amount of current provided to welding output 16 depends upon the point in time at which the SCRs are turned on after becoming forward-biased (i.e., firing angle) by a control circuit (not shown). When high power is required, conduction is started early in a half-cycle of the AC current signal. When low power is required, conduction is delayed until later in the half-cycle.
A large inductor 18 is used to filter the welding current. Inductor 18 integrates the voltage pulses from the SCR bridge according to the equation: e =L di/dt to reduce the peak-to-peak output ripple current. e is voltage measured in Volts, L is inductance measured in Henries, di is a change in current measured in Amps, and dt is a change in time measured in seconds. A freewheeling diode 20 is included to provide a conduction path for load current whenever the freewheeling path becomes forward-biased. Without this freewheeling path, the firing angle of the SCRs would be shifted forward, in order to balance the volt-seconds on inductor 18, resulting in increased peak-to-peak output ripple current and, therefore, inferior welding characteristics, particularly at low output current. Eliminating the freewheeling path also forces continuous current on the transformer secondary resulting in higher primary line current draw.
However, since freewheeling diode 20 is not used in the AC output mode, it requires a high current switch to connect it in the output circuit when DC output is used and remove it from the output circuit when AC output is used. This switch is costly and complicates the assembly process. This AC squarewave configuration is illustrated in FIG. 3. Thus, it is advantageous to construct a topography that provides the desired freewheeling function when DC output is used, and can be switched off electronically for AC output without adding diode 20 and the associated high current switch and wiring. FIG. 2 illustrates one such topography. In FIG. 2, SCRs 22 and 24 from the topography of FIG. 1 are removed and replaced with diodes 26 and 28. This circuit functions substantially the same as that of FIG. 1 without the need for a freewheeling diode because the freewheeling currents pass through diodes 26 and 28. However, when configured for an AC squarewave output, diodes 26 and 28 cannot block the freewheeling path and keep the SCR bridge in continuous conduction. Thus, the circuit of FIG. 2 cannot be used for both AC and DC operating modes.
Accordingly, there is a need for improved freewheeling current conduction in a welding power supply. Further, there is a need for a power conversion circuit for providing a welding power without the need for a freewheeling diode. Further still, there is a need for a power conversion circuit operable in both AC and DC operating modes without a freewheeling diode. Further yet, there is a need for a power conversion circuit that shares the burden of freewheeling current among several circuit components. The teachings hereinbelow extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the above needs.
According to an exemplary embodiment, a power circuit for a welding power supply includes a rectifier circuit and a control circuit. The rectifier circuit includes first and second switches. The control circuit is configured to provide a first control signal to the first switch at a predetermined firing angle to provide welding power through the first switch. The control circuit is configured to provide a second control signal to the second switch to allow freewheeling current to flow through the second switch.
According to another exemplary embodiment, a method of providing welding power from an AC input and conducting current through a freewheeling path of a power circuit includes controlling a first switch with a predetermined firing angle to pass a welding power therethrough and controlling a second switch to conduct current through the freewheeling path when the freewheeling path becomes forward biased.
According to yet another exemplary embodiment, a power conversion circuit for a welding power supply includes a means for providing a welding power based on a predetermined firing angle and a means for passing freewheeling current in response to a control signal.
According to still another exemplary embodiment, a welding power supply includes a transformer, a bridge rectifier and a control circuit. The transformer is coupled to an input power source and is configured to provide an input voltage. The bridge rectifier is coupled to the transformer for receiving the input voltage. The bridge rectifier has at least four SCRs. The control circuit is configured to fire a first SCR based upon a predetermined firing angle to pass a welding power therethrough. The control circuit is configured to fire a second SCR when a freewheeling conduction path becomes forward-biased.